In a system interconnection inverter that converts an output of a DC power source such as a solar power system or a fuel cell into an AC and interconnects the AC obtained by the conversion to a power system, for example, a leakage current flowing from the system interconnection inverter to the ground has to be suppressed to prevent an electric shock to a person or influence on other devices. An acceptable amount of the leakage current is defined in Electrical Appliances and Material Safety Act. Moreover, a test standard for a system interconnection device of a distributed-type power source is also defined by Japan Electrical Safety & Environment Technology Laboratories.
A solar power system has a floating capacitance between a terminal of a solar cell panel and a frame of the solar cell panel connected to the ground. Generally, an insulating layer made of a glass plate is formed on a surface of the solar cell panel, and the glass plate has a large plane surface. Thus, the floating capacitance increases when the rain wets the glass plate, whereby the leakage current also increases. There are several paths for the leakage current, one of which is a grounding wire of the inverter, for example. Increase in the floating capacity in one of the several paths leads to increase in the leakage current.
General methods for suppressing the leakage current includes a method of insulating the system interconnection inverter from the power system by means of an isolation transformer, and a method of using a common mode choke coil for suppressing the common mode current (see, for example, Non-Patent Document 1). Known methods besides these methods include a method of causing the common mode current to flow to an input side or the ground by bypassing with a filter (see, for example, Patent Document 1 and Non-Patent Document 1), a method of employing a two level pulse wide modulation (PWM) as the control scheme of the inverter and respectively outputting voltages of opposite polarity to upper and lower arms (see Patent Document 2), and a method that is a combination of these.
FIG. 1 is a block diagram showing a configuration of a solar power generation system interconnection inverter as a system interconnection inverter applied with a conventional countermeasure against leakage current. The solar power generation system interconnection inverter includes an inverter 1, an output filter 2, a first common mode choke coil 3a, a second common mode choke coil 3b, a first capacitor pair 41, a second capacitor pair 42, a solar cell 5, and a system transformer 7. It is to be noted that floating capacitances between the solar cell 5 and the ground are represented as a capacitor 6a and a capacitor 6b in FIG. 1.
The solar cell 5 generates DC power. The DC power generated by the solar cell 5 is supplied to the inverter 1 through the first common mode choke coil 3a. The first common mode choke coil 3a suppresses a flow of a common mode current from the inverter 1 to the solar cell 5.
The first capacitor pair 41, in which a capacitor 41a and a capacitor 41b are serially connected to each other, is disposed between input terminals a and b of the inverter 1 (between the output terminals of the first common mode choke coil 3a). A DC line positive voltage and a DC line negative voltage respectively appear at the points a and b. A DC line neutral point c, which is connected to the ground, is formed at the connection point of the capacitor 41a and the capacitor 41b. 
The inverter 1 is driven by the two level PWM control scheme. The inverter 1 converts the DC supplied from the solar cell 5 through the first common mode choke coil 3a into a PWM wave as shown in FIG. 2, for example, having amplitude varying from +1 to −1 and having a pulse wave whose pulse widths sequentially change, and transmits the PWM wave to the output filter 2.
The output filter 2 includes a reactor 21a whose input terminal is connected to one of the output terminals of the inverter 1, a reactor 21b whose input terminal is connected to the other output terminal of the inverter 1, and a capacitor 22 connected between the output terminals of the respective reactor 21a and reactor 21b. The output filter 2 converts the PWM signal outputted from the inverter 1 into a sine wave AC as shown in the dashed line in FIG. 2 and transmits the sine wave AC to the system transformer 7 through the second common mode choke coil 3b. 
The second capacitor pair 42, in which a capacitor 42a and a capacitor 42b are serially connected to each other, is disposed between input terminals d and e of the second common mode choke coil 3b (between the output terminals of the output filter 2). The sine wave AC (AC output signal) appears between the points d and e. An AC output neutral point f, which is connected to the ground, is formed at the connection point of the capacitor 42a and the capacitor 42b. 
The second common mode choke coil 3b suppresses a flow of the common mode current from the output filter 2 to the system transformer 7. The system transformer 7 transforms the sine wave AC supplied from the output filter 2 through the second common mode choke coil 3b and outputs the resultant current through power system terminals h for connection to a power system. The neutral point of the system transformer 7 is connected to the ground through a neutral point grounding wire i.
In the solar power generation system interconnection inverter having the above configuration, a high-frequency leakage current (common mode current) generated in the inverter 1 is prevented from flowing to the solar cell 5 side by the first common mode choke coil 3a, is prevented from flowing to the system transformer 7 side by the second common mode choke coil 3b, and thus flows into the ground by bypassing through the first capacitor pair 41 and the second capacitor pair 42.
Without any countermeasure against leakage current, i.e., without the first common mode choke coil 3a, the second common mode choke coil 3b, the first capacitor pair 41, and the second capacitor pair 42, the leakage current flows through a path of the neutral point grounding wire i of the system transformer 7→the ground→the floating capacitance 6 of the solar cell 5. In contrast, if the solar power generation system interconnection inverter described above drives the inverter 1 by use of the two level PWM control scheme, voltages of opposite polarity are always generated at the output terminals of the inverter 1. Thus, the voltage due to the common mode noise (hereinafter, referred to as common mode voltage) can be suppressed. Furthermore, as described above, the first common mode choke coil 3a suppresses the flow of the leakage current to the solar cell 5, the second common mode choke coil 3b suppresses the flow of the leakage current to the system transformer 7, and the first capacitor pair 41 and the second capacitor pair 42 cause the leakage current to flow into the ground by bypassing.